Intramolecular polymer complexes - viscosifiers for high ionic strength drilling fluids

ABSTRACT

A new family of terpolymers based on acrylamide/sodium styrene sulfonate/methacrylamidopropyltrimethylammonium chloride has been found to be an improved viscosity control additive for water-based drilling muds. The resultant muds display good viscosity characteristics, thermal stability and gel strength when formulated from the terpolymer intramolecular complex having the appropriate polymer concentration and salt level.

FIELD OF THE INVENTION

A new family of viscosification agents based on terpolymers ofacrylamide-sodium styrene sulfonatemethacrylamidopropyltrimethylammoniumchloride is described as an improved viscosity control additive forwater-based drilling muds. The present invention relates to theseterpolymer materials which function as viscosification agents when addedto water-based muds which are fluids used to maintain pressure, cooldrill bits and lift cuttings from the holes in the drilling operationfor oil and gas wells. The terpolymers have about 40 to 98 mole %acrylamide units, about 1 to about 50 mole % sodium styrene sulfonateunits and about 1 to about 50 mole %methacrylamidopropyltrimethylammonium chloride units. Normally, thelatter two units comprise less than 60 mole % of the total polymercomposition. A soluble, low molecular weight acid, base or salt can beadded to the aqueous mud solution, wherein the rheological properties ofthe drilling fluid is markedly enhanced.

The drilling muds formed from these polymeric materials exhibit improvedlow and high temperature rheological properties as compared to drillingmuds formed from homogeneous-charged polymers, i.e., polyelectrolytes.

BACKGROUND OF THE INVENTION

In the field of drilling in the exploration for oil and gas, animportant component is that of the formulation of drilling muds.Drilling muds are the fluids which are used to maintain pressure, cooldrill bits and lift cuttings from the holes, and vary in compositionover a wide spectrum. Generally, drilling muds are based on aqueousformulations or oil-based formulations.

A conventional water-based drilling mud formulation is comprised ofbasically the following ingredients: water, a clay such as bentonite,lignosulfonate, a weighting agent such as BaSO₄ (Barite), and a causticmaterial such as sodium hydroxide and a caustic material such as causticbarite, to adjust the pH of the drilling mud to a pH of about 10 toabout 10.5.

In addition to cooling the drill bit and sweeping out the drilling finesfrom the vicinity of bit, the muds are capable of imparting asubstantial positive pressure on a formation through its high density.This latter feature is due to the addition of high concentrations ofinsoluble, solid, high density particulates (i.e., weighting agents)such as barite. However, these particulates inhibit the drilling rateand possibly damage a variety of underground formations. This problembecomes even more acute as the drilling fines are "introduced" into themud. Therefore, there has been a substantial need for a homogeneous,high density drilling mud which exhibits good performance at both hightemperature and high ionic strength.

As alluded to previously, a very desirable change in the formulation ofa drilling fluid would be the elimination of all added particulates. Onepractical approach to this problem is to formulate a drilling fluid thatis clear, homogeneous, dense, single phase and possesses the appropriateviscosity requirements (in general, 40 to 50 cps). Therefore, awater-based mud containing principally a polymeric viscosifier in a highconcentration brine (weighting agent) could meet the above-statedrequirements. Such a fluid would be quite economical since someprocessing steps (and materials) are eliminated. For instance, brine canbe obtained directly at the drill site.

However, it should be pointed out that the ability of macromolecules toeffectively viscosify a high ionic strength solution is generally poor,since the dimensions of the polymer chains tend to collapse under theseconditions. This is especially true for polyelectrolytes (i.e.,homogeneous-charged polymers). A collapse in the dimensions of the chainresults in significant loss in viscosity. Therefore, it is imperativefor successful use of polymers in high ionic strength solutions thatchain expansion rather than contraction should take place. Polymericmaterials composed of acrylamide (AM), sodium styrene sulfonate (SSS)and methacrylamidopropyltrimethylammonium chloride (MAPTAC) wereobserved to enhance the viscosity of aqueous solutions containing highlevels of salt, acid or, base. (The specific details regarding synthesisand physical properties are found in Patent Application No. 478,657,filed Mar. 25, 1983, now U.S. Pat. No. 4,461,884.) These materials meetthe requirements for producing a homogeneous, single phase, highdensity, water-based drilling mud.

There has been substantial need for a water-based drilling fluid whichwould exhibit good performance at high temperature. Previous experiencehas shown that most polymeric viscosifiers are effective in salt-free(i.e., fresh water) systems; however, they lose their effectiveness uponthe addition of salt. As the temperature is increased, the viscosityloss becomes even more pronounced. There is need, therefore, for apolymeric viscosifier which can maintain viscosity and gel strength inhigh ionic strength, weighing agent-free (or at low concentrations),water-based muds up to high temperatures (exceeding 300° F.). Theseneeds are not adequately met by the current viscosifiers.

This invention describes an approach to viscosification of water-baseddrilling muds which permits the substitution of acrylamide-basedpolyampholyte terpolymers for amine clays and barite (weighting agent).The resulting polymer-modified drilling fluid displays rheologicalproperties which are in a desirable range for drilling mud applications,based on tests conducted for 16 hours at a variety of temperatures.

The types of acrylamide-based polyampholytes that are envisioned in thepresent invention include acrylamide as the nonionic monomer unit andthe following anionic and cationic species:

Anionic: 2-acrylamido-2-methylpropane sulfonic acid, sodium styrenesulfonate, (meth)acrylic acid, 2-sulfoethylmethacrylate and the like.

Cationic: Methacrylamidopropyltrimethylammonium chloride,dimethyldiallylammonium chloride, diethyldiallylammonium chloride,2-methacryloxy-2-ethyltrimethylammonium chloride,trimethylmethacryloxyethylammonium methosulfate,2-acrylamido-2-methylpropyltrimethylammonium chloride,vinylbenzyltrimethylammonium chloride and the like.

These monomers possess the appropriate water solubility so thatpolymerization can take place.

The preferred species of the instant invention is low to moderate chargedensity polyacrylamide-based polyampholytes with approximately 1 toabout 60 mole % ionic groups. A 1:1 molar ratio of anionic and cationicis not required for effective utilization of this polymer. It is foundthat these terpolymers are soluble (low charge density) or readilydispersible (moderate charge density) in fresh water systems.Homogeneous, clear solutions form with the addition of soluble acid,base, or salt showing that the polymer is readily soluble in thesesolutions. In addition, the viscosity increases with the addition ofthese solutes. As a consequence, these polymers are extremely effectiveviscosifiers in a high ionic strength, water-based mud, even atrelatively low levels. In the case of salt water, the concentration ofsalt can be from about 0.1 to about 50 grams per 100 grams of water.

SUMMARY OF THE INVENTION

A new family of viscosification agents based on terpolymers ofacrylamide-sodium styrene sulfonatemethacrylamidopropyltrimethylammoniumchloride is described an an improved viscosity control additive forwater-based drilling muds. The present invention relates to theseterpolymer materials which function as viscosification agents when addedto water-based muds which are fluids used to maintain pressure, cooldrill bits and lift cuttings from the holes in the drilling operationfor oil and gas wells. The terpolymers have about 40 to 98 mole %acrylamide units, about 1 to about 50 mole % sodium styrene sulfonateunits and about 1 to about 50 mole %methacrylamidopropyltrimethylammonium chloride units. Normally, thelatter two units comprise less than 60 mole % of the total polymercomposition. A soluble, low molecular weight acid, base or salt can beadded to the aqueous mud solution, wherein the rheological properties ofthe drilling fluid is markedly enhanced.

GENERAL DESCRIPTION

The present invention describes a new class of viscosification agentsfor water-based drilling muds which are used during operation of gas andoil wells, wherein these viscosification agents are intermolecularcomplexes, i.e., polyampholytes containing primarily acrylamide with lowto moderate concentrations of anionic and cationic groups. These lattertwo units are not necessarily present in a 1:1 molar ratio. Typically,the cationic monomer unit is methacrylamidopropyltrimethylammoniumchloride (MAPTAC) and the anionic monomer unit is sodium styrenesulfonate (SSS). However, many water soluble anionic and cationicmonomer units can be substituted for MAPTAC and SSS units. It is theplacement of these oppositely-charged species onto the polymer chainthat imparts substantially different physical properties to thesematerials, as compared to homogeneous-charged macromolecules, i.e.,polyelectrolytes.

The water-based drilling muds of the instant invention minimallycomprise, but can also include other additives, if necessary, an aqueousliquid such as fresh water or salt water; a clay such as bentonite;lignosulfonate as a viscosifier; a weighting material such as barite(BaSO₄), and a caustic material such as a sodium hydroxide or lime addedto adjust pH to about 10.0 to 10.5. In general, the specific gravity isabout 7 pounds per gallon to about 20 pounds per gallon, more preferablyabout 10 to 18, and most preferably about 12 to about 16.

A typical, but non-limiting example of a caustic material which can bereadily employed is sodium hydroxide or lime.

A typical, but non-limiting example of a suitable clay additive isbentonite.

A typical, but non-limiting examples of a weighting agent which can bereadily employed is barite or a barium sulfate, which may optionally besurface treated with a variety of other cations, such as calcium.

The terpolymers are formed by a free radical copolymerization process.The principal monomer used in the free radical aqueous solutioncopolymerization process is acrylamide monomer, which is copolymerizedwith an anionic monomer (typically, sodium styrene sulfonate) and acationic monomer (typically, methacrylamidopropyltrimethylammoniumchloride).

A typical water-based drilling mud, as evisioned by the instantinvention, comprises water or salt water; weighting material necessaryto give the desired mud density; about 0.25 to about 15 lb/bbl. of theacrylamide/sodium styrenesulfonate/methacrylamidopropyltrimethylammonium chloride intrapolymercomplex; and sufficient concentration of base to adjust the pH of thewater-based drilling mud to about 10.0 to about 10.5. Higher levels ofthe sulfonated polymer can be employed but it is not normallyeconomically attractive. The drilling mud may also contains a clay suchas Bentonite, at a concentration level of about 4 to about 30 lb/bbl.,wherein the clay is added to the drilling mud to promote circulation andimprove hole stability and cleaning. A lignosulfonate, which is adeflocculation agent, can be added to the drilling mud at aconcentration level of about 1 to about 30 lb/bbl.

Alternatively, a typical water-based drilling mud, as envisioned by theinstant invention, comprises water in which sufficient salt (such asnon-chloride, iron bromide and calcium bromide) is dissolved to give thedesired mud density, and about 0.25 to about 5 lb/bbl of theacrylamide-MAPTAC-SSS-terpolymer. Higher levels of the terpolymer can beemployed, but, it is not economically attractive. The mud may contain asufficient concentration of base to adjust the pH of the water-based mudto its appropriate level (normally 10.0 to 10.5). The attractive featureof this mud is the elimination of high concentrations of insoluble,solid, high density particulates (example, weighting agents). In someinstances, these particulates inhibit the drilling process throughdamage of the underground formation and reduction in the drilling rate.

The terpolymers of the instant invention are formed by a free radicalterpolymerization process in an aqueous medium of an acrylamide monomer,a sodium styrene sulfonate monomer and amethacrylamidopropyltrimethylammonium chloride monomer. The resultantwater soluble terpolymer has the formula: ##STR1## wherein x is about 40to about 98 mole %, more preferably about 50 to about 95, and mostpreferably about 80 to about 90, y is about 1 to about 50 mole %, morepreferably about 2 to about 20 and most preferably about 5 to about 10,and z is about 1 to about 50 mole %, more preferably about 2 to about20, and most preferably about 5 to about 10, wherein y and z are lessthan 60 mole %, and M is an amine, or a metal cation selected from thegroup consisting of aluminum, iron, lead, Groups IA, IIA, IB and IIB ofthe Periodic Table of Elements wherein the terpolymer has about 1 toabout 50 mole % of charged monomer units.

The molecular weight as derived from intrinsic viscosities, for theterpolymers of acrylamide/sodium styrenesulfonate/methacrylamidopropyltrimethylammonium chloride is about 10³ toabout 5×10⁶, more preferably about 10⁴ to about 2×10⁶ and mostpreferably about 10⁵ to about 10⁶. The means for determining themolecular weights of the water soluble terpolymers from the viscosity ofsolutions of the terpolymers compresses the initial isolation of thewater soluble terpolymers, purification and redissolving the terpolymersin water to give solutions with known concentrations. The flow times ofthe solutions and the pure solvent were measured in a standardUbbelholde viscometer. Subsequently, the reduced viscosity is calculatedthrough standard methods utilizing these values. Extrapolation to zeropolymer concentration leads to the intrinsic viscosity of the polymersolution. The intrinsic viscosity is directly related to the molecularweight through the well-known Mark Houwink relationship.

The water soluble terpolymers of acrylamide/sodium styrenesulfonate/methacrylamidopropyltrimethylammonium chloride are formed by aconventional free radical terpolymerization in an aqueous medium whichcomprises the steps of forming a reaction solution of acrylamidemonomer, sodium styrene sulfonate monomer andmethacrylamidopropyltrimethylammonium chloride monomer (50 wt. %solution in water) in distilled water, wherein the total monomerconcentration is about 1 to about 40 grams of total monomer per 100grams of water, more preferably about 5 to about 30, and most preferablyabout 10 to about 20; purging the reaction solution with nitrogen;adding sufficient acid to the reaction solution to adjust the pH of thereaction solution to about 4.5 to 5; heating the reaction solution to atleast 55° C. while maintaining the nitrogen purge, adding sufficientfree radical initiator to the reaction solution at 55° C. to initiateterpolymerization of the acrylamide monomer, the sodium styrenesulfonate monomer, and the methacrylamidopropyltrimethylammoniumchloride monomer; terpolymerizing said monomers of acrylamide, sodiumstyrene sulfonate and methacrylamidopropyltrimethylammonium chloride ata sufficient temperature and for a sufficient period of time to formsaid water soluble terpolymer; and recovering said water solubleterpolymer from said reaction solution.

In general, the acrylamide, anionic and cationic monomers are dissolvedin a water phase in the presence of an initiator, wherein thetemperature is sufficient to initiate polymerization. The resultantterpolymer is added to the drilling mud formulation at about 0.5 toabout 20 lb/bbl.

The total concentration of monomers in the water is about 1 to about 40grams of total monomer per 100 grams of water, more preferably about 5to about 30, and most preferably about 10 to about 20. Terpolymerizationof the acrylamide monomer, sodium styrene sulfonate monomer, andmethacrylamidopropyltrimethylammonium chloride monomer is effected at atemperature of about 30 to about 90, more preferably at about 40 toabout 70, and most preferably at about 50 to about 60 for a period oftime of about 1 to about 24 hours, more preferably about 3 to about 10,and most preferably about 4 to about 8.

A suitable method of recovery of the formed water soluble terpolymerfrom the aqueous reaction solution comprises precipitation in acetone,methanol, ethanol and the like.

Suitable free radical initiators for the free radical terpolymerizationof the acrylamide monomers, the sodium styrene sulfonate monomer, andthe methacrylamidopropyltrimethylammonium chloride monomer are selectedfrom the group consisting of potassium persulfate, benzoyl peroxide,hydrogen peroxide, azobisisobutyronitrile and the like. Theconcentration of the free radical initiator is about 0.001 to about 2.0grams of free radical initiator per 100 grams of total monomer, morepreferably about 0.01 to about 1.0, and most preferably about 0.05 toabout 0.1.

It should be pointed out that neither the mode of polymerization(solution, suspension, or emulsion polymerization technique and thelike) nor the initiator is critical, provided that the method or theproduct of the initiation step does not inhibit production of thepolyampholyte or chemically modify the initial molecular structure ofthe reacting monomers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate the present invention, without;however, limiting the same hereto.

EXAMPLE 1

Into a 1 liter--4 necked flash was added:

13.8 g. MAPTAC (50% water solution),

40.0 g. acrylamide,

6.44 g. sodium styrene sulfonate, and

300 ml. distilled water.

The pH of the solution was adjusted to 4.5 to 5.0 with 20% phosphonicacid solution. The solution was purged with nitrogen gas for 1 hour toremove dissolved oxygen. As the nitrogen gas purging began, the solutionwas heated to 55° C. At this point, 0.1 g. potassium persulfate wasadded to the solution. After 4 hours, the polymer was precipitated fromsolution with acetone. Subsequently, the resulting polymer was washedseveral times with a large excess of acetone and dried in a vacuum ovenat 60° C. for 24 hours.

Elemental analysis shows that this polyampholyte or intramolecularcomplex contains 90.5 mole % acrylamide, 1.86 mole % sodium styrenesulfonate, and 7.64 mole % methacrylamidopropyltrimethylammoniumstyrenesulfonate complex.

Table I shows the effectiveness of the acrylamide-based polyampholyte asa viscosifier and gel strength additive for particulate-free,water-based muds. The data show that muds containing salt can be treatedwith the intrapolymer complex to enhance both the viscosity and gelstrength characteristics. These desirable properties are maintainedafter subjecting the mud to high temperature (400° F.). In addition, thedata show that the acrylamide-based polyampholyte is not very effectiveas a viscosifier and gel strength additive for water-based mudscontaining fresh water.

                  TABLE I                                                         ______________________________________                                        Rheological Properties of Several Particulate-Free Water-Based                Muds Containing an Acrylamide-Based Polyampholyte                             De-                                                                           sig- Weight                                                                   na-  Per-                                      Temp,                          tion cent.sup.(a)                                                                          Water   600.sup.(b)                                                                        300.sup.(c)                                                                        PV.sup.(d)                                                                         YP.sup.(e)                                                                         Gel.sup.(f)                                                                         °F.                     ______________________________________                                        1A   1.5     Fresh   19   11    8   3    4-4    70                            1B   1.5     Fresh   16   10    6   4    2-3   150                            1C   1.5     Fresh   23   12   11   1    4-4   400                            2A   1.5     Salt.sup.(g)                                                                          85   53   32   21   4-4    70                            2B   1.5     Salt.sup.(g)                                                                          80   50   30   20   4-4   150                            2C   1.5     Salt.sup.(g)                                                                          32   17   15   2    4-4   400                            ______________________________________                                         .sup.(a) Polymer concentration.                                               .sup.(b) Reading taken on a Fann ® rotational viscometer at 600 RPM.      .sup.(c) Reading obtained on a Fann ® rotational viscometer at 300        RPM.                                                                          .sup.(d) Plastic viscosity in centipose units: Difference between the 600     RPM and 300 RPM measurements.                                                 .sup.(e) Yield point in units of lbs/100 ft..sup.2 : Difference between       300 RPM measurement and plastic viscosity.                                    .sup.(f) First number is the initial value (expressed as lbs/100              ft..sup.2) on the viscometer at 3 RPM, while the second number correspond     to the value after 10 minutes has elapsed.                                    .sup.(g) Sodium chloride  10 lb/gal.                                     

EXAMPLE 2

Particulate-free, water-based muds were prepared using conventionallaboratory techniques. The muds contain fresh or salt (sodium chloride)water and 1.5 wt. % acrylamide-based polyampholyte described inExample 1. The muds were aged at 70° F. to ensure physical homogeneity.Aliquots of the mud were aged in pressurized cells at 150° and 400° F.for 4 hours. The cells were cooled at room temperature, depressurized,and then the rheological properties of the mud were measured on a FannModel 35 viscometer at 115° F. Muds containing 0.0 and 10 lb/gal ofsodium chloride are presented in Table I.

Table II shows again the effectiveness of the acrylamide-basedpolyampholyte as a viscosifier and gel strength additive forparticulate-free, water-based muds. The data show that muds containingsalt (calcium chloride) can be treated with the intrapolymer complex toenhance both the viscosity and gel strength characteristics. Thesedesirable properties are maintained after subjecting the mud to hightemperature (400° F.). In addition, the data show that theacrylamide-based polyampholyte is not very effective as a viscosifierand gel strength additive for water-based muds containing fresh water.Furthermore, the data indicates that the polyampholyte performs to abetter extent in calcium chloride solutions than in sodium chloridesolutions. The improved performance is due essentially to the polymer'sincreased hydrodynamic volume in the calcium chloride mud system.Undoubtedly, the macromolecular complex is more compatible in the lattermud system, which is a very desirable feature.

                  TABLE II                                                        ______________________________________                                        Rheological Properties of Several Particulate-Free Water-Based                Muds Containing an Acrylamide-Based Polyampholyte                             De-                                                                           sig- Weight                                                                   na-  Per-                                      Temp,                          tion cent.sup.(a)                                                                          Water   600.sup.(b)                                                                        300.sup.(c)                                                                        PV.sup.(d)                                                                         YP.sup.(e)                                                                         Gels.sup.(f)                                                                        °F.                     ______________________________________                                        1A   1.5     Fresh   19   11    8    3   4-4    70                            1B   1.5     Fresh   16   10    6    4   2-3   150                            1C   1.5     Fresh   23   12   11    1   4-4   400                            3A   1.5     Salt.sup.(g)                                                                          242  150  92   58   9-7    70                            3B   1.5     Salt.sup.(g)                                                                          226  139  87   52   5-6   150                            3C   1.5     Salt.sup.(g)                                                                          96   54   42   12   5-7   400                            4A   0.75    Salt.sup.(g)                                                                          87   51   36   15   3-3    70                            4B   0.75    Salt.sup.(g)                                                                          88   49   39   10   3-5   150                            4C   0.75    Salt.sup.(g)                                                                          31   19   12    5   4-4   400                            ______________________________________                                         .sup.(a) Polymer Concentration.                                               .sup.(b) Reading taken on a Fann ® rotational viscometer at 600 RPM.      .sup.(c) Reading obtained on a Fann ® rotational viscometer at 300        RPM.                                                                          .sup.(d) Plastic viscosity in centipose units: Difference between the 600     RPM and 300 RPM measurements.                                                 .sup.(e) Yield point in units of lbs/100 ft..sup.2 : Difference between       the 300 RPM measurement and plastic viscosity                                 .sup.(f) First number is the initial value (expressed as lbs/100              ft..sup.2) on the viscometer at 3 RPM, while the second number correspond     to the value after 10 minutes has elapsed.                                    .sup.(g) Calcium chloride  11.3 lbs/gal.                                 

Finally, we observe a marked improvement in the rheological propertiesof the muds with an increase in the polymer concentration. This isreadily observed in a direct comparison of 3A-C with 4A-C.

What is claimed is:
 1. A drilling mud which comprises:(a) salt waterhaving about 0.1 to about 50 grams of salt per 100 ml. of water; (b)about 4 to about 30 lbs/bbl of clay; (c) about 1 to about 30 lbs/bbl ofa lignosulfonate; (d) weighting material of sufficient quantitynecessary to achieve the desired density; (e) about 0.25 to about 5lbs/bbl of a terpolymer having the formula: ##STR2## wherein x is about40 to about 98 mole %, y is about 1 to about 50 mole %, z is about 1 toabout 50 mole percent, wherein y and z are less than 60 mole %, and M isselected from the group consisting of amines and a metallic cation beingselected from the group consisting of lead, iron, aluminum, Groups IA,IIA, IB and II of the Periodic Table of Elements, the molecular weightof said terpolymer as measured by intrinsic viscosity being 1×10³ to5×10⁶ ; and (f) base of sufficient quantity to adjust the pH of thewater-based drilling mud to about 10 to about 10.5.
 2. A water-baseddrilling mud according to claim 1 wherein said clay is Bentonite.
 3. Awater-based drilling mud according to claim 1 wherein said base issodium hydroxide.
 4. A water-based drilling mud according to claim 1wherein M is sodium.
 5. A drilling mud which comprises:(a) salt waterhaving about 0.1 to about 50 grams of salt per 100 grams of water; and(b) about 0.25 to about 15 lbs/bbl of the acrylamide-based polyampholytecontaining low to moderate levels of anionic and cationic groups toincrease the viscosity of said drilling mud, said terpolymer containingacrylamide monomer units and having about 1 to about 50 mole % of thecharged monomer units, said terpolymer possessing unequal molar ratio ofanionic and cationic monomers, said anionic monomer being sodium styrenesulfonate, and said cationic monomer being selected from the groupconsisting of methacrylamidopropyltrimethylammonium chloride,dimethyldiallylammonium chloride, diethyldiallylammonium chloride,2-methacryloxy-2-ethyltrimethylammonium chloride,trimethylmethacryloxethylammonium methosulfate,2-acrylamido-2-methylpropyltrimethylammonium chloride, andvinylbenzyltrimethylammonium chloride.